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Earth: a fast-moving Spacecraft

In fact, we’re all living in a fast-moving spacecraft named Earth. Our planet is moving around our sun in an elliptical (an ellipse which is very close to a circle) orbit. The Sun is (our solar system, the Sun, Earth and all the other planets and objects) whirls around the center of our galaxy. And, our galaxy and the other galaxies in our neighborhood are also rushing towards a structure called the Great Attractor, a region of space roughly 150 million light-years (one light year is about six trillion miles) away from us. This Great Attractor, having a mass 100 quadrillion times greater than our sun and span of 500 million light-years, is made of both the visible matter that we can see along with the so-called dark matter that we cannot see.

So, even when you’re resting on your armchair, you’re flying through space faster than the fastest human-made object. But how fast actually?

Questions about how fast the earth, or anything, for that matter, is moving are incomplete unless they also ask, “Compared to what?” Without a frame of reference, questions about motion cannot be completely answered. For example, when we say a car is traveling at 120 km/h, we mean that it travels 120 km across the Earth in one hour and ignore the fact that the Earth itself is moving.

Earth’s rotation

The earth is 24,900 miles in circumference at the equator, or 40,000 kilometers. The earth takes 24 hours to make one rotation. So:

24,900 / 24 = 1,037 mi/h or 1,666 km/h

That number is the greatest number at the equator, and as you move toward the poles it decreases. At the poles, the speed is zero and you are simply rotating in place at one rotation every 24 hours. If you are sitting somewhere in South Florida moving at about 1,000 miles per hour or 1,610 km/h.

Earth’s orbital speed around Sun

The Earth in different points in its orbit. (sizes and distances not to scale)

Earth’s orbit is the path in which the Earth travels around the Sun. Earth lies at an average distance of 149.59787 million kilometers (93 million miles) from the Sun and a complete orbit occurs every 365.256 days (1 sidereal year, which is the orbital period of the earth around the sun, taking the stars as a reference frame. It is 20 minutes longer than the tropical year because of precession), during which time Earth travels 940 million kilometers (584 million miles).

584,000,000 / (365×24) ~= 67,000 mph

Earth’s orbital speed averages about 30 km/s (67,000 mph), which is fast enough to cover the planet’s diameter in seven minutes and the distance to the Moon in four hours.

The Sun’s Motion

View of a Milky Way look-alike galaxy, NGC-6744

The Solar System is near the edge of the Milky Way and orbiting around the galactic core. The solar system is something like 25,000 light years away from the center of the galaxy. By measuring the speed that other galaxies are moving towards or away from us, we can get a reasonable idea of our own orbital speed: the Milky Way makes one rotation every 250 million years or so. That gives the solar system a speed of something like 490,000 mph or around 790,000 kph. But we can’t just add this to Earth’s speed around the Sun, because we’re moving in circles – all we can say is that Earth’s speed around the galaxy is somewhere between 423,000 mph and 557,000 mph (681,000 kph-896,000 kph) depending on the day of the year.

The speed of the Milky Way itself

Today, some 12 to 15 billion years after the Big Bang, there has been a lot of stretching. Space has expanded quite a bit. The flash of the Big Bang has stretched until it is now much longer, lower energy waves: microwaves and other radio waves. But the waves have stretched with the space they occupy, and so they still fill the universe, just the way they did at the time of creation.

Astronomers call the collection of all these stretched waves the “Cosmic Background Radiation” or CBR. Physicists back in the late 1940’s predicted that there should be such a background, but since no one had the equipment to find it, the prediction was forgotten. Then, in the mid 1960s, two scientists working for Bell Laboratories, Arno Penzias and Robert Wilson, accidentally discovered the CBR while helping to get communications satellite technology going for the phone company. After astronomers used other telescopes and rockets in orbit to confirm that the radio waves the two scientists had discovered were really coming from all over space, Penzias and Wilson received the Nobel Prize in physics for having found the most direct evidence for the Big Bang.

The CBR provides a “frame of reference” for the universe at large, relative to which we can measure our motion. Because the CBR permeates all space, we can finally answer the original question fully, using the CBR as the frame of reference.

In 1989, the COBE satellite was placed in orbit about the earth to measure the long-diluted radiation echo of the birth of our universe (CBR). One of COBE’s discoveries was that the earth was moving with respect to this CBR with a well-defined speed and direction.

The speed turns out to be an astounding 1.3 million miles per hour (2.1 million km/hr)! We are moving roughly in the direction on the sky that is defined by the constellations of Leo and Virgo. Although the reasons for this motion are not fully understood, astronomers believe that there is a huge concentration of matter in this direction. Some people call it The Great Attractor, although we now know that the pull is probably not due to one group of galaxies but many. Still the extra gravity in this direction pulls the Milky Way (and many neighbor galaxies) in that direction.

The Hubble Telescope turned its lens to the region of the sky where the Great Attractor is located.